This invention relates to an apparatus for screening material onto a workpiece and, more particularly, relates to an apparatus for sequentially screening two different types of material onto a workpiece.
Extrusion and mesh (or silk) screening are the two major techniques in the electronics industry today for personalizing ceramics. Extrusion screening is achieved by mechanically forcing paste from a reservoir through a nozzle onto a metallic mask/stencil. Mesh screening utilizes a polyurethane-based nozzle chamfered at approximately a 45 degree angle to both push the paste along and through a silk mesh/stencil. Apparatus for screening is described in Davis et al. U.S. Pat. No. 4,362,486, the disclosure of which is incorporated by reference herein.
Glass, ceramic, glass ceramic and aluminum nitride (hereafter just ceramic) structures are used in the production of electronic substrates and devices for electronics packaging applications. Many different types of structures can be used. For example, a multilayered ceramic circuit substrate may comprise patterned metal layers which act as electrical conductors sandwiched between ceramic layers which act as insulators. The substrates are designed with termination pads for attaching semiconductor chips, capacitors, resistors, connection leads, pins, solder balls, solder columns etc. Interconnection between buried conductor levels can be achieved through vias formed by metal paste-filled holes in the individual ceramic layers formed prior to lamination, which, upon sintering will become a sintered dense metal interconnection of metal based conductor.
The termination pads are often multi-layered stacks of metallization and are conventionally produced with multiple screenings, with the underlying layer being screened and dried before application of another mask and screening and drying of the next layer. Greenstein U.S. Pat. No. 4,025,669, Siuta U.S. Pat. No. 5,202,153, and Knickerbocker et al. U.S. Pat. No. 5,293,504, the disclosures of which are incorporated by reference herein, are examples where multiple screenings have been utilized to obtain either a thicker layer or a multiple layer stack of metallization.
While the prior art shows the individual layers of the stack to be perfectly aligned with every other layer in the stack, the reality is very far from this ideal case. For example, Natarajan et al. U.S. Pat. No. 5,639,562, the disclosure of which is incorporated by reference herein, shows a two layer composite metal pad with both layers perfectly aligned.
Gaynes et al. U.S. Pat. No. 5,565,033, the disclosure of which is incorporated by reference herein, discloses a process for making thicker layers of solder pastes and conductive adhesives. In one embodiment, a flexible TEFLON material (TEFLON, a tetrafluoroethylene (TFE) compound, is a trademark of E.I. duPont de Nemours & Co.) is uses as the trailing lip of the screening nozzle. Gaynes et al. recognizes the disadvantages of multiple screenings, namely, contamination between successively screened layers and the time associated with two passes through screening and drying.
In practice, the individual layers may be shifted from the layer above or below it. Mitani et al. U.S. Pat. No. 4,324,815, the disclosure of which is incorporated by reference herein, recognizes the positional error that can occur with each printing step. As disclosed in Mitani et al., the bottom layer could be made larger than the top layer so that the top layer is "captured" by the bottom layer.
However, with the trend to increasing the density of the termination pads (and reducing the spacing between adjoining termination pads), it is no longer possible to oversize the bottom layer to capture the top layer.
Disclosed in IBM Technical Disclosure Bulletin, 28, No. 2, pp. 808-809 (July 1985), the disclosure of which is incorporated by reference herein, is a conventional screening apparatus for screening a paste onto ceramic greensheets. The reservoir for the disclosed apparatus only contains a single paste. Thus, each time a layer of paste is deposited, the paste dispenser needs to be changed before the next layer of paste could be deposited. Obviously, changing the paste dispenser after each layer of paste is deposited is time consuming and ultimately adds to the manufacturing cost of producing the finished part.
Cornell et al. IBM Technical Disclosure Bulletin, 14, No. 3, p. 739 (August 1971), the disclosure of which is incorporated by reference herein, discloses an extrusion screening nozzle having a TEFLON material tip for uniform wiping of the screening paste from the mask.
The art is replete with apparatus for dispensing two different kinds of materials. Typically, these prior art apparatus dispense the two different kinds of material simultaneously and/or side-by-side each other. Examples of such apparatus are Nicholls U.S. Pat. No. 3,827,602, Speer U.S. Pat. No. 4,040,420, Speer U.S. Pat. No. 4,261,481, Kido et al. U.S. Pat. No. 4,801,434, Fiedler et al. U.S. Pat. No. 4,949,874, Fischer U.S. Pat. No. 5,290,259 and Smolen, Jr. et al. U.S. Pat. No. 5,683,014, the disclosures of which are incorporated by reference herein.
It would be desirable to be able to sequentially lay down two different layers of paste without having to change the paste dispenser. It would further be desirable to be able to lay down two different layers of paste on top of each other without having to change the paste dispenser.
Accordingly, it is a purpose of the present invention to have an apparatus that is capable of sequentially laying down two different layers of paste without having to change the paste dispenser.
It is another purpose of the present invention to have an apparatus that is capable of laying down two different layers of paste in complete registration with each other without having to change the paste dispenser.